Using bitcoinx.com:

power cost = .15 USD/ kWh
a power consumption of 350 W for a 50,000 MH/s miner
at a cost of $2,500
profitability decline per year of .61 per year (standard value in calculator)


Point where break even on investment is no longer possible:

@ 50 USD per BTC = Difficulty 123,000,000

Net profit per year: -13.60 USD


@ 100 USD per BTC = Difficulty 240,000,000

Net profit over 1 year: -0.58 USD

@ 150 USD per BTC = Difficulty 368,000,000

Net profit over 1 year : -5.46 USD

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Point where power costs more than revenue (not counting initial cost of miner):


USD/BTC=50. Difficulty: 920,000,000

Power cost per year: 394.47 USD
Revenue per year: 393.58 USD

USD/BTC=100. Difficulty: 1,900,000,000

Power cost: 394.47 USD
Revenue: 381.28 USD

USD/BTC=150. Difficulty: 2,500,000,000

Power cost: 460.22 USD
Revenue: 434.79 USD